electric-fusion is generally used for connecting pipes, and such electric-fusion of pipes will be briefly described below.
In the bonding of pipes made of polyethylene or polypropylene to be used for tap water pipes and other purposes, heat fusion processes have been basically used to obtain a completely hermetic seal.
Conventional heat fusion processes include butt fusion welding, heat fusion socket welding, electro-fusion socket welding, and so on.
Butt fusion welding is also referred to as butt welding, in which end faces of pipes are butted against each other without the use of an additional socket. In detail, butt fusion welding is carried out according to the following steps of: fixing the pipes with hydraulic or pneumatic clamps in such a way that the end faces of the pipes to be welded face each other, smoothing the end faces to be welded, placing a heating plate between the end faces, tightly pressing the end faces to be welded against the heating plate via hydraulic or pneumatic pressure, heating the heating plate to a suitable melting point (e.g., about 210° C. for polyethylene) to fuse the end faces of the pipes, removing the heating plate quickly after the fusion, tightly pressing the fused end faces of the pipes against each other, and cooling the pipes while maintaining them in the pressed position for a predetermined time, so that the fused portions of the pipes are solidified, thereby bonding the pipes together.
This process bonds the pipes by melting pipe materials without the use of an additional socket. However, this process requires a hot welder, which is heavy and thus difficult to carry to a working site, and furthermore, has a complicated structure.
Another process uses a common socket, in which the inner periphery of the socket and the outer periphery of a pipe are melted simultaneously and then bonded together. That is, the outer periphery of the pipe and the inner periphery of the socket are heated at the same time so that the welding surface of each is melted simultaneously, and when they are heated sufficiently, the inner periphery of the socket is quickly fitted onto the outer periphery of the pipe and is thus in tight contact therewith. Then, the resultant structure is cooled down for a predetermined time in this tight contact state.
A further process is electro socket fusion. This process uses a socket having a heating element such as a heating coil inserted therein. The socket is injection-molded with the heating element integrally disposed therein. The heating coil may include, for example, wires of Cu, Cu alloys, nichrome, or the like, which have a specific degree of electric resistance. The socket may be fabricated, for example, by coiling an electric resistance wire coated with a thermoplastic resin, connecting electric terminals to ends of the wire, inserting the wire into a mold, and molding a resin integrally with the wire. After the socket fabricated in this way is fitted onto a pipe to be fused, electric current is applied to the socket for a predetermined time by a simple power supply so that the outer periphery of the pipe and the inner periphery of the socket are melted simultaneously and bonded together.
Such electro socket fusion is also referred to as electric fusion or electro fusion.
electric-fusion has been used generally in connector pipes.
At present, electro-fusion connector pipes are preferred by users because they are easy to use, and various heating elements for the electro-fusion connector pipes are being developed.
In addition to the foregoing approaches, electro-fusion is also disclosed in Korean Patent Application Publication No. 10-2002-0012809, entitled “Electronic Fusion Sheet of Thermoplastic (e.g., Polyethylene and Polypropylene) Pipes by Printed (Coated) Heating Circuit,” Korean Patent Application Publication No. 10-2004-069622, entitled “Heat Generating Bushing for Plastic Pipes,” Korean Patent Application Publication No. 10-2004-0096757, entitled “Fusion Structure for Plastic Pipes,” Korean Patent Application Publication No. 10-2005-0003231, entitled “Fusion Structure for Plastic Pipes Using Fusion Ring” and so on, which are incorporated hereby in their entirety for reference.
However, current electro-fusion connector pipes have the following problems:
(1) In the case of fusion bonding, the surfaces to be bonded should be free from impurities in order to realize airtightness or watertightness. The impurities on the bonding surfaces act as an interface, obstructing bonding of melting parts. Thus, the impurities are a factor that degrades bonding efficiency. To prevent such a problem, the surfaces to be bonded are cleaned prior to being fusion bonded. However, some impurities in the form of a thin film are rarely cleaned.
(2) In the bonding by fusion, it is important to bond two parts under a suitable degree of pressure. (For example, melting parts are bonded together while being tightly pressed against each other in the case of butt fusion, in which a pressing process is performed hydraulically or pneumatically.) In traditional electro fusion process, natural heat expansion is the only resource that generates a bonding pressure and an artificial pressure from outside has not been considered since the heating element is installed inside the connector pipe.
(3) However, in order to use the pressure owing to the natural expansion, a very small clearance is required between the inner periphery of the electro-fusion connector pipe and the outer periphery of the plastic pipe, inserted into the connector pipe. Precision machining, required to minimize the clearance, also raises manufacturing costs.